Missile Alternate (flak) bug in mult ...

I noticed a weird bug tonight when my roommate and I were playing Hidden Gulch on endless.  At first, I thought he had also chosen and was using the missile laungher, but it turns out he wasn't.  I was puzzled then as to why, almost randomly, flak explosions would appear out of thin air and wipe out the fliers... not that I was complaining!
 
It seems that flak explosions will reappear many times (doing damage in all occurrances!) where they've been shot at before, but only when an enemy gets close enough.  I didn't do a very thorough amount of testing, but it appears that this only occurs in multiplayer.  I haven't tried other maps yet, but I suspect the results will be the same regardless of the map.
 
As always, I've included a video showing the "problem"... though to be honest I'll be a little sad to see this one go!
 
http://youtu.be/b4Hm2xxL9W8